Number of pairs whose LCM = 800 and HCF = 20
Let the numbers be x and y. Now Highest Common Factor = 20 implies numbers are
a = 20 * x
b = 20 * y
Least Common Multiple = 800 implies 20 * x * y = 800
x * y = 40
Number of values (x,y) such that x*y = 40 and HCF of (x,y) = 1 because when taking LCM there was no common factor left between x and y are (1,40) (5,8)
Corrosponding pairs are (20,800) (100,160) = 2